Here are some book recommendations for learning Koine Greek, the dialect that the New Testament was written in:

1) The Basics of Biblical Greek, by William Mounce (probably the most commonly known)
2) Complete Vocabulary Guide to the Greek New Testament, by Warren Trenchard (a great source to learn and memorize vocab)
3) The Morphology of Biblical Greek, by William Mounce (a great resource to help identify all those morphological forms)
4) Greek Grammar Beyond the Basics, by Daniel Wallace (this is next on my list!)